2012 05 03_machine_learning_lecture_09

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2012 05 03_machine_learning_lecture_09

  1. 1. Metric Learning ICML 2010 Tutorial Brian Kulis University of California at Berkeley June 21, 2010Brian Kulis University of California at Berkeley Metric Learning
  2. 2. IntroductionLearning problems with distances and similarities k-means Support vector machines k-nearest neighbors Most algorithms that employ kernel methods Other clustering algorithms (agglomerative, spectral, etc) ... Brian Kulis University of California at Berkeley Metric Learning
  3. 3. Choosing a distance function Brian Kulis University of California at Berkeley Metric Learning
  4. 4. Choosing a distance functionExample: UCI Wine data set 13 features 9/13 features have mean value in [0, 10] 3/13 features have mean value in [10, 100] One feature has a mean value of 747 (with std 315) Using a standard distance such as Euclidean distance, the largest feature dominates the computation That feature may not be important for classification Need a weighting of the features that improves classification or other tasks Brian Kulis University of California at Berkeley Metric Learning
  5. 5. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  6. 6. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  7. 7. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  8. 8. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  9. 9. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  10. 10. Metric Learning as Learning Transformations Feature re-weighting Learn weightings over the features, then use standard distance (e.g., Euclidean) after re-weighting Diagonal Mahalanobis methods (e.g., Schultz and Joachims) Number of parameters grows linearly with the dimensionality d Full linear transformation In addition to scaling of features, also rotates the data For transformations from d dimensions to d dimensions, number of parameters grows quadratically in d For transformations to r < d dimensions, this is linear dimensionality reduction Non-linear transformation Variety of methods Neural nets Kernelization of linear transformations Complexity varies from method to method Brian Kulis University of California at Berkeley Metric Learning
  11. 11. Supervised vs Unsupervised Metric Learning Unsupervised Metric Learning Dimensionality reduction techniques Principal Components Analysis Kernel PCA Multidimensional Scaling In general, not the topic of this tutorial... Supervised and Semi-supervised Metric Learning Constraints or labels given to the algorithm Example: set of similarity and dissimilarity constraints This is the focus of the tutorial Brian Kulis University of California at Berkeley Metric Learning
  12. 12. Themes of the tutorial Not just a list of algorithms General principles Focus on a few key methods Recurring ideas Scalability Linear vs non-linear Online vs offline Optimization techniques utilized Statements about general formulations Applications Where is metric learning applied? Success stories Limitations Brian Kulis University of California at Berkeley Metric Learning
  13. 13. Outline of Tutorial Motivation Linear metric learning methods Mahalanobis metric learning Per-example methods Non-linear metric learning methods Kernelization of Mahalanobis methods Other non-linear methods Applications Conclusions Brian Kulis University of California at Berkeley Metric Learning
  14. 14. Mahalanobis Distances Brian Kulis University of California at Berkeley Metric Learning
  15. 15. Mahalanobis Distances Assume the data is represented as N vectors of length d: X = [x1 , x2 , ..., xN ] Squared Euclidean distance 2 d(x1 , x2 ) = x1 − x2 2 T = (x1 − x2 ) (x1 − x2 ) Let Σ = i,j (xi − µ)(xj − µ)T The “original” Mahalanobis distance: dM (x1 , x2 ) = (x1 − x2 )T Σ−1 (x1 − x2 ) Brian Kulis University of California at Berkeley Metric Learning
  16. 16. Mahalanobis Distances Equivalent to applying a whitening transform Brian Kulis University of California at Berkeley Metric Learning
  17. 17. Mahalanobis Distances Assume the data is represented as N vectors of length d: X = [x1 , x2 , ..., xN ] Squared Euclidean distance 2 d(x1 , x2 ) = x1 − x2 2 T = (x1 − x2 ) (x1 − x2 ) Mahalanobis distances for metric learning Distance parametrized by d × d positive semi-definite matrix A: dA (x1 , x2 ) = (x1 − x2 )T A(x1 − x2 ) Used for many existing metric learning algorithms[Xing, Ng, Jordan, and Russell; NIPS 2002][Bar-Hillel, Hertz, Shental, and Weinshall; ICML 2003][Bilenko, Basu, and Mooney; ICML 2004][Globerson and Roweis; NIPS 2005][Weinberger, Blitzer, and Saul; NIPS 2006] Brian Kulis University of California at Berkeley Metric Learning
  18. 18. Mahalanobis Distances dA (x1 , x2 ) = (x1 − x2 )T A(x1 − x2 ) Why is A positive semi-definite (PSD)? If A is not PSD, then dA could be negative Suppose v = x1 − x2 is an eigenvector corresponding to a negative eigenvalue λ of A dA (x1 , x2 ) = (x1 − x2 )T A(x1 − x2 ) = vT Av = λvT v = λ < 0 Brian Kulis University of California at Berkeley Metric Learning
  19. 19. Mahalanobis Distances Properties of a metric: d(x, y) ≥ 0 d(x, y) = 0 if and only if x = y d(x, y) = d(y, x) d(x, z) ≤ d(x, y) + d(y, z) dA is not technically a metric Analogous to Euclidean distance, need the square root: dA (x1 , x2 ) = (x1 − x2 )T A(x1 − x2 ) Square root of the Mahalanobis distance satisfies all properties if A is strictly positive definite, but if A is positive semi-definite then second property is not satisfied Called a pseudo-metric In practice, most algorithms work only with dA Brian Kulis University of California at Berkeley Metric Learning
  20. 20. Mahalanobis Distances Can view dA as the squared Euclidean distance after applying a linear transformation Decompose A = G T G via Cholesky decomposition (Alternatively, take eigenvector decomposition A = V ΛV T and look at A = (Λ1/2 V T )T (Λ1/2 V T )) Then we have dA (x1 , x2 ) = (x1 − x2 )T A(x1 − x2 ) = (x1 − x2 )G T G (x1 − x2 ) = (G x1 − G x2 )T (G x1 − G x2 ) 2 = G x1 − G x2 2 Mahalanobis distance is just the squared Euclidean distance after applying the linear transformation G Brian Kulis University of California at Berkeley Metric Learning
  21. 21. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  22. 22. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  23. 23. Example: Four Blobs Want to learn: 1 0 1 0 A= G= √ 0 0 Brian Kulis University of California at Berkeley Metric Learning
  24. 24. Example: Four Blobs Brian Kulis University of California at Berkeley Metric Learning
  25. 25. Example: Four Blobs Want to learn: √ 0 0 A= G= 0 1 0 1 Brian Kulis University of California at Berkeley Metric Learning
  26. 26. Drawbacks to Mahalanobis Metric Learning Memory overhead grows quadratically with the dimensionality of the data Does not scale to high-dimensional data (d = O(106 ) for many image embeddings) Only works for linearly separable data Cannot seemingly be applied to “real” data! These drawbacks will be discussed later Brian Kulis University of California at Berkeley Metric Learning
  27. 27. Metric Learning Problem Formulation Typically 2 main pieces to a Mahalanobis metric learning problem A set of constraints on the distance A regularizer on the distance / objective function In the constrained case, a general problem may look like: minA r (A) s.t. ci (A) ≤ 0 0 ≤ i ≤ C A 0 r is a regularizer/objective on A and ci are the constraints on A An unconstrained version may look like: C min r (A) + λ ci (A) A 0 i=1 Brian Kulis University of California at Berkeley Metric Learning
  28. 28. Defining Constraints Similarity / Dissimilarity constraints Given a set of pairs S of points that should be similar, and a set of pairs of points D of points that should be dissimilar A single constraint would be of the form dA (xi , xj ) ≤ for (i, j) ∈ S or dA (xi , xj ) ≥ u for (i, j) ∈ D Easy to specify given class labels Relative distance constraints Given a triple (xi , xj , xk ) such that the distance between xi and xj should be smaller than the distance between xi and xk , a single constraint is of the form dA (xi , xj ) ≤ dA (xi , xk ) − m, where m is the margin Popular for ranking problems Brian Kulis University of California at Berkeley Metric Learning
  29. 29. Defining Constraints Aggregate distance constraints Constrain the sum of all pairs of same-class distances to be small, e.g., yij dA (xi , xj ) ≤ 1 ij where yij = 1 if xi and xj are in the same class, and 0 otherwise Other constraints Non-parametric probability estimation constraints Constraints on the generalized inner product xT Axj : i dA (xi , xj ) = xT Axi + xT Axj − 2xT Axj i j i Brian Kulis University of California at Berkeley Metric Learning
  30. 30. Defining the Regularizer or Objective Loss/divergence functions Squared Frobenius norm: A − A0 2 F LogDet divergence: tr(AA−1 ) − log det(AA−1 ) − d 0 0 General loss functions D(A, A0 ) Will discuss several of these later Other regularizers A 2F tr(AC0 ) (i.e., if C0 is the identity, this is the trace norm) Brian Kulis University of California at Berkeley Metric Learning
  31. 31. Choosing a Regularizer Depends on the problem! Example 1: tr(A) Trace function is the sum of the eigenvalues Analogous to the 1 penalty, promotes sparsity Leads to low-rank A Example 2: LogDet Divergence Defined only over positive semi-definite matrices Makes computation simpler Possesses other desirable properties Example 3: A 2 F Arises in many formulations Easy to analyze and optimize Brian Kulis University of California at Berkeley Metric Learning
  32. 32. Defining the Optimization Many existing Mahalanobis distance learning methods can be obtained simply by choosing a regularizer/objective and constraints We will discuss properties of several of these Brian Kulis University of California at Berkeley Metric Learning
  33. 33. Xing et al.’s MMCProblem posed as follows: max (xi ,xj )∈D dA (xi , xj ) A s.t. c(A) = (xi ,xj )∈S dA (xi , xj ) ≤ 1 A 0. Here, D is a set of pairs of dissimilar pairs, S is a set of similar pairs Objective tries to maximize sum of dissimilar distances Constraint keeps sum of similar distances small Use square root in regularizer to avoid trivial solution[Xing, Ng, Jordan, and Russell; NIPS 2002] Brian Kulis University of California at Berkeley Metric Learning
  34. 34. Xing et al.’s MMCAlgorithm Based on gradient descent over the objective followed by an iterative projection step to find a feasible A Constraint c(A) is linear in A, can be solved cheaply Orthogonal projection onto A 0 achieved by setting A’s negative eigenvalues to 0 Iterative between these two steps to find feasible A for both constraints, then take a step in the gradient of the objective Despite relative simplicity, the algorithm is fairly slow (many eigenvalue decompositions required) Does not scale to large problems Objective and constraints only look at the sums of distances Brian Kulis University of California at Berkeley Metric Learning
  35. 35. Schultz and JoachimsProblem formulated as follows: min A 2 F A s.t. dA (xi , xk ) − dA (xi , xj ) ≥ 1 ∀(i, j, k) ∈ R A 0. Constraints in R are relative distance constraints There may be no solution to this problem; introduce slack variables min A 2 +γ F (i,j,k)∈R ξijk A,ξ s.t. dA (xi , xk ) − dA (xi , xj ) ≥ 1 − ξijk ∀(i, j, k) ∈ R ξijk ≥ 0 ∀(i, j, k) ∈ R A 0.[Schultz and Joachims; NIPS 2002] Brian Kulis University of California at Berkeley Metric Learning
  36. 36. Schultz and JoachimsAlgorithm Key simplifying assumption made A = M T DM, where M is assumed fixed and known and D is diagonal dA (xi , xj ) = (xi − xj )T A(xi − xj ) = (xi − xj )T M T DM(xi − xj ) = (Mxi − Mxj )T D(Mxi − Mxj ) Effectively constraining the optimization to diagonal matrices Resulting optimization problem is very similar to SVMs, and resulting algorithm is similar By choosing M to be a matrix of data points, method can be kernelized Fast algorithm, but less general than full Mahalanobis methods Brian Kulis University of California at Berkeley Metric Learning
  37. 37. Kwok and TsangProblem formulated as follows: 2 CS CD min A F + NS (xi ,xj )∈S ξij + ND (xi ,xj )∈D ξij − CD γν A,ξ,γ s.t. dI (xi , xj ) ≥ dA (xi , xj ) − ξij ∀(xi , xj ) ∈ S dA (xi , xj ) − dI (xi , xj ) ≥ γ − ξij ∀(xi , xj ) ∈ D ξij ≥ 0 γ≥0 A 0. Same regularization as Schultz and Joachims Similarity/dissimilarity constraints instead of relative distance constraints No simplifying assumptions made about A[Kwok and Tsang; ICML 2003] Brian Kulis University of California at Berkeley Metric Learning
  38. 38. Neighbourhood Components AnalysisProblem formulated as follows: exp(−dA (xi ,xj )) max i j∈Ci ,j=i P exp(−dA (xi ,xk )) A k=i s.t. A 0. Ci is the set of points in the same class as point xi (not including xi )Motivation Minimize the leave-one-out KNN classification error LOO error function is discontinuous Replace by a softmax; each point xi chooses a nearest neighbor xj based on probability exp(−dA (xi , xj )) pij = k=i exp(−dA (xi , xk ))[Goldberger, Roweis, Hinton, and Salakhutdinov; NIPS 2004] Brian Kulis University of California at Berkeley Metric Learning
  39. 39. Neighbourhood Components AnalysisAlgorithm Problem is non-convex Rewrite in terms of G , where A = G T G Eliminates A 0 constraint Run gradient descent over GProperties Easy to control the rank of A: just optimize over low-rank G Simple, unconstrained optimization No guarantee of global solution Brian Kulis University of California at Berkeley Metric Learning
  40. 40. MCMLRecall NCA probabilities exp(−dA (xi , xj )) pij = k=i exp(−dA (xi , xk )) 0 Introduce an “ideal” probability distribution pij : 0 1 if i and j from same class pij ∝ 0 otherwise. Minimize divergence between p 0 and p: min KL(p 0 , p) A s.t. A 0.[Globerson and Roweis; NIPS 2005] Brian Kulis University of California at Berkeley Metric Learning
  41. 41. MCMLProperties Unlike NCA, MCML is convex Global optimization possible Algorithm based on optimization over the dual Similar to Xing: gradient step plus projection Not discussed in detail in this tutorial Brian Kulis University of California at Berkeley Metric Learning
  42. 42. A Closer Look at Some Algorithms As can be seen, several objectives are possible We will take a look at 3 algorithms in-depth for their properties POLA An online algorithm for learning metrics with provable regret Also the first supervised metric learning algorithm that was shown to be kernelizable LMNN Very popular method Algorithm scalable to billions of constraints Extensions for learning multiple metrics ITML Objective with several desirable properties Simpler kernelization construction Online variant Brian Kulis University of California at Berkeley Metric Learning
  43. 43. POLASetup Estimate dA in an online manner Also estimate a threshold b At each step t, observe tuple (xt , xt , yt ) yt = 1 if xt and xt should be similar; -1 otherwise Consider the following loss t (A, b) = max(0, yt (dA (xt , xt )2 − b) + 1) Hinge loss, margin interpretation Appropriately update A and b each iteration[Shalev-Shwartz, Singer, and Ng; NIPS 2004] Brian Kulis University of California at Berkeley Metric Learning
  44. 44. Online Learning Setup Define the total loss to be T L= t (At , bt ) t=1 Online learning methods compare the loss to the best fixed, offline predictor A∗ and threshold b ∗ Define regret for T total timesteps as T T ∗ ∗ RT = t (At , bt ) − t (A , b ) t=1 t=1 Design an algorithm that minimizes the regret Same setup as in other online algorithms (classification, regression) √ Modern optimization methods achieve O( T ) regret for a general class, and O(log T ) for some special cases Brian Kulis University of California at Berkeley Metric Learning
  45. 45. POLA Algorithm Consider the following convex sets Ct = {(A, b) | t (A, b) = 0} Ca = {(A, b) | A 0, b ≥ 1} Consider orthogonal projections: 2 PC (x) = argminy∈C x − y 2 Think of (A, b) as a vector in d 2 + 1 dimensional space Each step, project onto Ct , then project onto Ca Brian Kulis University of California at Berkeley Metric Learning
  46. 46. POLA Algorithm Projection onto Ct Let vt = xt − xt Then projection is computed in closed form as t (At , bt ) αt = vt 4 + 1 2 ˆ At = At − yt αt vt vT t ˆ bt = bt + αt yt Projection onto Ca Orthogonal projection onto positive semi-definite cone obtained by setting negative eigenvalues to 0 ˆ But, update from At to At was rank-one Only 1 negative eigenvalue (interlacing theorem) Thus, only need to compute the smallest eigenvalue and eigenvector (via a power method or related) and subtract it off Brian Kulis University of California at Berkeley Metric Learning
  47. 47. Analysis of POLA Theorem: Let (x1 , x1 , y1 ), ..., (xT , xT , yT ) be a sequence of examples and let R be such that ∀t, R ≥ xt − xt 4 + 1. Assume there exists an 2 A∗ 0 and b ∗ ≥ 1 such that t (A∗ , b ∗ ) = 0 ∀t. Then the following bound holds for all T ≥ 1: T t (At , bt ) 2 ≤ R( A∗ 2 F + (b ∗ − b1 )2 ). t=1 Note that since t (A∗ , b ∗ ) = 0 for all t, the total loss is equal to the regret Can generalize this to a regret bound in the case when t (A∗ , b ∗ ) does not always equal 0 Can also run POLA in batch settings Brian Kulis University of California at Berkeley Metric Learning
  48. 48. POLA in Context Can we think of POLA in the framework presented earlier? Yes—regularizer is A 2 and constraints are defined by the hinge loss F t Similar to both Schultz and Joachims, and Kwok and Tsang We will see later that POLA can also be kernelized to learn non-linear transformations In practice, POLA does not appear to be competitive with current state-of-the-art Brian Kulis University of California at Berkeley Metric Learning
  49. 49. LMNN Similarly to Schultz and Joachims, utilize relative distance constraints A constraint (xi , xj , xk ) ∈ R has the property that xi and xj are neighbors of the same class, and xi and xk are of different classes[Weinberger, Blitzer, and Saul; NIPS 2005] Brian Kulis University of California at Berkeley Metric Learning
  50. 50. LMNN Problem Formulation Also define set S of pairs of points (xi , xj ) such that xi and xj are neighbors in the same class Want to minimize sum of distances of pairs of points in S Also want to satisfy the relative distance constraints Mathematically: min (xi ,xj )∈S dA (xi , xj ) A s.t. dA (xi , xk ) − dA (xi , xj ) ≥ 1 ∀(xi , xj , xk ) ∈ R A 0. Brian Kulis University of California at Berkeley Metric Learning
  51. 51. LMNN Problem Formulation Also define set S of pairs of points (xi , xj ) such that xi and xj are neighbors in the same class Want to minimize sum of distances of pairs of points in S Also want to satisfy the relative distance constraints Mathematically: min (xi ,xj )∈S dA (xi , xj ) + γ (xi ,xj ,xk )∈R ξijk A,ξ s.t. dA (xi , xk ) − dA (xi , xj ) ≥ 1 − ξijk ∀(xi , xj , xk ) ∈ R A 0, ξijl ≥ 0. Introduce slack variables Brian Kulis University of California at Berkeley Metric Learning
  52. 52. Comments on LMNN Algorithm Special-purpose solver Relies on subgradient computations Ignores inactive constraints Example: MNIST—3.2 billion constraints in 4 hours Software available Performance One of the best-performing methods Works in a variety of settings Brian Kulis University of California at Berkeley Metric Learning
  53. 53. LMNN ExtensionsLearning with Multiple Local Metrics Learn several local Mahlanobis metrics instead a single global one Cluster the training data into k partitions Denote ci as the corresponding cluster for xi Learn k Mahalanobis distances A1 , ..., Ak Formulation min (xi ,xj )∈S dAcj (xi , xj ) A s.t. dAck (xi , xk ) − dAcj (xi , xj ) ≥ 1 ∀(xi , xj , xk ) ∈ R Ai 0 ∀i. Introduce slack variables as with standard LMNN[Weinberger and Saul; ICML 2008] Brian Kulis University of California at Berkeley Metric Learning
  54. 54. LMNN Results Results show improvements using multiple metrics Weinberger and Saul also extend LMNN to use ball trees for fast search No time to go into details, see paper Brian Kulis University of California at Berkeley Metric Learning
  55. 55. ITML and the LogDet Divergence We take the regularizer to be the Log-Determinant Divergence: D d (A, A0 ) = trace(AA−1 ) − log det(AA−1 ) − d 0 0 Problem formulation: minA D d (A, A0 ) s.t. (xi − xj )T A(xi − xj ) ≤ u if (i, j) ∈ S [similarity constraints] (xi − xj )T A(xi − xj ) ≥ if (i, j) ∈ D [dissimilarity constraints][Davis, Kulis, Jain, Sra, and Dhillon; ICML 2007] Brian Kulis University of California at Berkeley Metric Learning
  56. 56. LogDet Divergence: Properties D d (A, A0 ) = trace(AA−1 ) − log det(AA−1 ) − d, 0 0 Properties: Scale-invariance D d (A, A0 ) = D d (αA, αA0 ), α>0 In fact, for any invertible M D d (A, A0 ) = D d (M T AM, M T A0 M) Expansion in terms of eigenvalues and eigenvectors (A = V ΛV T , A0 = UΘU T ): λi λi D d (A, A0 ) = (vT uj )2 i − log −d θj θj i,j Brian Kulis University of California at Berkeley Metric Learning
  57. 57. Existing Uses of LogDet Information Theory Differential relative entropy between two same-mean multivariate Gaussians equal to LogDet divergence between covariance matrices Statistics LogDet divergence is known as Stein’s loss in the statistics community Optimization BFGS update can be written as: min D d (B, Bt ) B subject to B st = yt (“Secant Equation”) st = xt+1 − xt , yt = ft+1 − ft Brian Kulis University of California at Berkeley Metric Learning
  58. 58. Key Advantages Simple algorithm, easy to implement in Matlab Method can be kernelized Scales to millions of data points Scales to high-dimensional data (text, images, etc.) Can incorporate locality-sensitive hashing for sub-linear time similarity searches Brian Kulis University of California at Berkeley Metric Learning
  59. 59. The Metric Learning Problem D d (A, A0 ) = trace(AA−1 ) − log det(AA−1 ) − d 0 0 ITML Goal: minA D d (A, A0 ) s.t. (xi − xj )T A(xi − xj ) ≤ u if (i, j) ∈ S [similarity constraints] (xi − xj )T A(xi − xj ) ≥ if (i, j) ∈ D [dissimilarity constraints] Brian Kulis University of California at Berkeley Metric Learning
  60. 60. Algorithm: Successive Projections Algorithm: project successively onto each linear constraint — converges to globally optimal solution Use projections to update the Mahalanobis matrix: min D d (A, At ) A s.t. (xi − xj )T A(xi − xj ) ≤ u Can be solved by O(d 2 ) rank-one update: At+1 = At + βt At (xi − xj )(xi − xj )T At Advantages: Automatic enforcement of positive semidefiniteness Simple, closed-form projections No eigenvector calculation Easy to incorporate slack for each constraint Brian Kulis University of California at Berkeley Metric Learning
  61. 61. Recent work in Mahalanobis methods Recent work has looked at other regularizers, such as tr(A), which learns low-rank matrices Improvements in online metric learning (tighter bounds) Kernelization for non-linear metric learning, the topic of the next section Brian Kulis University of California at Berkeley Metric Learning
  62. 62. LEGO Online bounds proven for a variant of POLA based on LogDet regularization Combines the best of both worlds Minimize the following function at each timestep ft (A) = D d (A, At ) + ηt t (A, xt , yt ) At is the current Mahalanobis matrix ηt is the learning rate t (A, xt , yt ) is a loss function, e.g. 1 t (A, xt , yt ) = (dA (xt , yt ) − p)2 2 √ For appropriate choice of step size, can guarantee O( T ) regret Empirically outperforms POLA significantly in practice[Jain, Kulis, Dhillon, and Grauman; NIPS 2009] Brian Kulis University of California at Berkeley Metric Learning
  63. 63. Non-Mahalanobis methods: Local distance functions General approach Learn a distance function for every training data point ij Given m features per point, denote dm as the distance between the m-th feature in points xi and xj j Denote wm as a weight for feature m of point xj Then the distance between an arbitary (e.g., test) image xi and a training image xj is M j ij d(xi , xj ) = w m dm m=1 At test time Given test image xi , compute d(xi , xj ) between xi and every training point xj Sort distances to find nearest neighbors[Frome, Singer, Sha, and Malik; ICCV 2007] Brian Kulis University of California at Berkeley Metric Learning
  64. 64. Non-Mahalanobis methods: Local distance functionsOptimization framework j Denote wj as the vector of weights wm As before, construct triples (i, j, k) of points such that the distance between xi and xj should be smaller than the distance between xi and xk Formulate the following problem: min wj 2 j 2 W s.t. d(xi , xk ) − d(xi , xj ) ≥ 1 ∀(xi , xj , xk ) ∈ R wj ≥ 0 ∀j. Brian Kulis University of California at Berkeley Metric Learning
  65. 65. Non-Mahalanobis methods: Local distance functionsOptimization framework j Denote wj as the vector of weights wm As before, construct triples (i, j, k) of points such that the distance between xi and xj should be smaller than the distance between xi and xk Formulate the following problem: min wj 2 +γ j 2 (i,j,k) ξijk W s.t. d(xi , xk ) − d(xi , xj ) ≥ 1 − ξijk ∀(xi , xj , xk ) ∈ R wj ≥ 0 ∀j. Introduce slack variables as before Very similar to LMNN and other relative distance methods! Brian Kulis University of California at Berkeley Metric Learning
  66. 66. Non-Mahalanobis methods: Local distance functions Schultz and Joachims min A 2 F A s.t. dA (xi , xk ) − dA (xi , xj ) ≥ 1 ∀(i, j, k) ∈ R A 0. Frome et al. min wj 2 j 2 W s.t. d(xi , xk ) − d(xi , xj ) ≥ 1 ∀(xi , xj , xk ) ∈ R wj ≥ 0 ∀j. Brian Kulis University of California at Berkeley Metric Learning
  67. 67. Linear Separability No linear transformation for this grouping Brian Kulis University of California at Berkeley Metric Learning
  68. 68. Kernel Methods Map input data to higher-dimensional “feature” space: x → ϕ(x) Idea: Run machine learning algorithm in feature space Use the following mapping: 2   x1 √ x1 x= →  2x1 x2  x2 2 x2 Brian Kulis University of California at Berkeley Metric Learning
  69. 69. Mapping to Feature Space Brian Kulis University of California at Berkeley Metric Learning
  70. 70. Kernel Methods Map input data to higher-dimensional “feature” space: x → ϕ(x) Idea: Run machine learning algorithm in feature space Use the following mapping: 2   x1 √ x1 x= →  2x1 x2  x2 2 x2 Kernel function: κ(x, y) = ϕ(x), ϕ(y) “Kernel trick” — no need to explicitly form high-dimensional features In this example: ϕ(x), ϕ(y) = (xT y)2 Brian Kulis University of California at Berkeley Metric Learning
  71. 71. Kernel Methods: Short Intro Main idea Take an existing learning algorithm Write it using inner products Replace inner products xT y with kernel functions ϕ(x)T ϕ(y) If ϕ(x) is a non-linear function, then algorithm has been implicitly non-linearly mapped Examples of kernel functions κ(x, y) = (xT y)p Polynomial Kernel x−y 2 2 κ(x, y) = exp − Gaussian Kernel 2σ 2 κ(x, y) = tanh(c(xT y) + θ) Sigmoid Kernel Kernel functions also defined over objects such as images, trees, graphs, etc. Brian Kulis University of California at Berkeley Metric Learning
  72. 72. Example: Pyramid Match Kernel Compute local image features Perform an approximate matching between features of two images Use multi-resolution histograms View as a dot product between high-dimensional vectors[Grauman and Darrell, ICCV 2005] Brian Kulis University of California at Berkeley Metric Learning
  73. 73. Example: k-meansRecall the k-means clustering algorithm Repeat until convergence: Compute the means of every cluster πc 1 µc = xi |πc | x ∈π i c Reassign points to their closest mean by computing 2 x − µc 2 for every data point x and every cluster πcKernelization of k-means Expand x − µc 2 as 2 T T 2 xi ∈πc xT xi xi ,xj ∈πc xi xj x x− + |πc | |πc |2 No need to explicitly compute the mean; just compute this for every point to every cluster Brian Kulis University of California at Berkeley Metric Learning
  74. 74. Example: k-meansRecall the k-means clustering algorithm Repeat until convergence: Compute the means of every cluster πc 1 µc = xi |πc | x ∈π i c Reassign points to their closest mean by computing 2 x − µc 2 for every data point x and every cluster πcKernelization of k-means Expand x − µc 2 as 2 2 κ(x, xi ) xi ,xj ∈πc κ(xi , xj ) xi ∈πc κ(x, x) − + |πc | |πc |2 Replace inner products with kernels, and this is kernel k-means While k-means finds linear separators for the cluster boundaries, kernel k-means finds non-linear separators Brian Kulis University of California at Berkeley Metric Learning
  75. 75. Distances vs. Kernel Functions Mahalanobis distances: dA (x, y) = (x − y)T A(x − y) Inner products / kernels: κA (x, y) = xT Ay Algorithms for constructing A learn both measures Brian Kulis University of California at Berkeley Metric Learning
  76. 76. From Linear to Nonlinear LearningConsider the following kernelized problem You are given a kernel function κ(x, y) = ϕ(x)T ϕ(y) You want to run a metric learning algorithm in kernel space Optimization algorithm cannot use the explicit feature vectors ϕ(x) Must be able to compute the distance/kernel over arbitrary points (not just training points) Mahalanobis distance is of the form: dA (x, y) = (ϕ(x) − ϕ(y))T A(ϕ(x) − ϕ(y)) Kernel is of the form: κA (x, y) = ϕ(x)T Aϕ(y) Can be thought of as a kind of kernel learning problem Brian Kulis University of California at Berkeley Metric Learning
  77. 77. Kernelization of ITML First example: ITML Recall the update for ITML At+1 = At + βt At (xi − xj )(xi − xj )T At Distance constraint over pair (xi , xj ) βt computed in closed form How can we make this update independent of the dimensionality? Brian Kulis University of California at Berkeley Metric Learning
  78. 78. Kernelization of ITML Rewrite the algorithm in terms of inner products (kernel functions) At+1 = At + βt At (xi − xj )(xi − xj )T At Inner products in this case: xT At xj i Brian Kulis University of California at Berkeley Metric Learning
  79. 79. Kernelization of ITML Rewrite the algorithm in terms of inner products (kernel functions) X T At+1 X = X T At X + βt X T At X (ei − ej )(ei − ej )T X T At X Entry (i, j) of X T At X is exactly xT Axj = κA (xi , xj ) i Denote X T At X as Kt , the kernel matrix at step t Kt+1 = Kt + βt Kt (ei − ej )(ei − ej )T Kt Brian Kulis University of California at Berkeley Metric Learning
  80. 80. Kernel Learning Squared Euclidean distance in kernel space: 2 xi − xj 2 = xT xi + xT xj − 2xT xj i j i Replace with kernel functions / kernel matrix: κ(xi , xi ) + κ(xj , xj ) − 2κ(xi , xj ) = Kii + Kjj − 2Kij Related to ITML, define the following optimization problem minK D d (K , K0 ) s.t. Kii + Kjj − 2Kij ≤ u if (i, j) ∈ S [similarity constraints] Kii + Kjj − 2Kij ≥ if (i, j) ∈ D [dissimilarity constraints] K0 = X T X is the input kernel matrix To solve this, only the original kernel function κ(xi , xj ) is required Brian Kulis University of California at Berkeley Metric Learning
  81. 81. Kernel Learning Bregman projections for the kernel learning problem: Kt+1 = Kt + βt Kt (ei − ej )(ei − ej )T Kt Suggests a strong connection between the 2 problems Theorem: Let A∗ be the optimal solution to ITML, and A0 = I . Let K ∗ be the optimal solution to the kernel learning problem. Then K ∗ = X T A∗ X . Solving the kernel learning problem is “equivalent” to solving ITML So we can run entirely in kernel space But, given two new points, how to compute distance?[Davis, Kulis, Jain, Sra, and Dhillon; ICML 2007] Brian Kulis University of California at Berkeley Metric Learning
  82. 82. Induction with LogDet Theorem: Let A∗ be the optimal solution to ITML, and let A0 = I . Let K ∗ be the optimal solution to the kernel learning problem, and let K0 = X T X be the input kernel matrix. Then A∗ = I + XSX T −1 −1 S = K0 (K ∗ − K0 )K0 Gives us a way to implicitly compute A∗ once we solve for K ∗ Algorithm Solve for K ∗ Construct S using K0 and K ∗ Given two points x and y, the kernel κA (x, y) = xT Ay is computed as n κA (xi , xj ) = κ(xi , xj ) + Sij κ(x, xi )κ(xj , y) i,j=1[Davis, Kulis, Jain, Sra, and Dhillon; ICML 2007] Brian Kulis University of California at Berkeley Metric Learning
  83. 83. Kernelization of POLA Recall updates for POLA ˆ At = At − yt αt vt vTt At+1 ˆ = At − λd ud uT d vt is the difference of 2 data points ˆ ud is the smallest eigenvector of At 1st update projects onto set Ct where hinge loss is zero (applied only when loss is non-zero) ˆ 2nd update projects onto PSD cone Ca (applied only when At has negative eigenvalue) Claim: Analogous to ITML, A∗ = XSX T , where X is the matrix of data points Prove this inductively Brian Kulis University of California at Berkeley Metric Learning
  84. 84. Kernelization of POLAProjection onto Ct At = XSt X T Say the 2 data points are indexed by i and j Then vt = X (ei − ej ) ˆ Rewrite At − yt αvt vT to get update from St to St : t ˆ At = XSt X T − yt αt X (ei − ej )(ei − ej )T X T = X (St − yt αt (ei − ej )(ei − ej )T )X TProjection onto Ca ˆ ud is an eigenvector of At , i.e., ˆ At ud ˆ = X St X T u = λd ud 1 ˆ T ud = X St X ud = X q λd Construction for q non-trivial; involves kernelized Gram-Schmidt Expensive (cubic in dimensionality) Brian Kulis University of California at Berkeley Metric Learning
  85. 85. General Kernelization Results Recent work by Chatpatanasiri et al. has shown additional kernelization results for LMNN Neighbourhood Component Analysis Discriminant Neighborhood Embedding Other recent results show additional, general kernelization results Xing et al. Other regularizers (trace-norm) At this point, most/all existing Mahalanobis metric learning methods can be kernelized Brian Kulis University of California at Berkeley Metric Learning
  86. 86. Kernel PCA Setup for principal components analysis (PCA) Let X = [x1 , ..., xn ] be a set of data points Typically assume data is centered, not critical here Denote SVD of X as X = U T ΣV Left singular vectors in U corresponding to non-zero singular values are an orthonormal basis for the span of the xi vectors Covariance matrix is C = XX T = U T ΣT ΣU, kernel matrix is K = X T X = V T ΣT ΣV Standard PCA recipe Compute SVD of X Project data onto leading singular vectors U, e.g., ˜ = Ux x Brian Kulis University of California at Berkeley Metric Learning
  87. 87. Kernel PCA Key result from the late 1990s: kernelization of PCA Can also form projections using the kernel matrix Allows one to avoid computing SVD If X = U T ΣV , then U = Σ−1 VX T Ux = Σ−1 VX T x Computation involves inner products X T x, eigenvectors V of the kernel matrix, and eigenvalues of the kernel matrix Relation to Mahalanobis distance methods Kernel PCA allows one to implicitly compute an orthogonal basis U of the data points, and to project arbitrary data points onto this basis For a data set of n points, dimension of basis is at most n Projecting onto U results in an n-dimensional vector Brian Kulis University of California at Berkeley Metric Learning
  88. 88. Using kernel PCA for metric learning Given a set of points in kernel space X = [ϕ(x1 ), ..., ϕ(xn )] Form a basis U and project data onto that basis to form ˜ X = [˜1 , ..., xn ] = [Uϕ(x1 ), ...., Uϕ(xn )] using kernel PCA x ˜ Consider a general unconstrained optimization problem f that is a function of kernel function values, i.e. f ({ϕ(xi )T Aϕ(xj )}n ) i,j=1 Associated minimization min f ({ϕ(xi )T Aϕ(xj )}n ) i,j=1 A 0 Theorem: The optimal value of the above optimization is the same as that of min f ({˜iT A xj }n ) x ˜ i,j=1 A 0 where A is n × n.[Chatpatanasiri, Korsrilabutr, Tangchanachaianan, and Kijsirikul; ArXiV 2008] Brian Kulis University of California at Berkeley Metric Learning
  89. 89. Consequences Any Mahalanobis distance learning method that is unconstrained and can be expressed as a function of learned inner products can be kernelized Examples Neighbourhood Components Analysis LMNN (write as unconstrained via the hinge loss) Discriminant neighborhood embedding Generalizing to new points For a new point ϕ(x), construct ˜ and use Mahalanobis distance with x learned matrix A Algorithms Exactly the same algorithms employed as in linear case Brian Kulis University of California at Berkeley Metric Learning
  90. 90. Extensions Chatpatanasiri et al. considered extensions for low-rank transformations Also showed benefits of kernelization in several scenarios Recent results (Jain et al.) have shown complementary results for constrained optimization problems ITML is a special case of this analysis Other methods follow easily, e.g., methods based on trace-norm regularization Now most Mahalanobis metric learning methods have been shown to be kernelizable Brian Kulis University of California at Berkeley Metric Learning
  91. 91. Scalability in Kernel Space In many situations, dimensionality d and the number of data points n is high Typically, linear Mahalanobis metric learning methods scale as O(d 2 ) or O(d 3 ) Kernelized Mahalanobis methods scale as O(n2 ) or O(n3 ) What to do when both are large? Main idea: restrict the basis used for learning the metric Can be applied to most methods Brian Kulis University of California at Berkeley Metric Learning
  92. 92. Scalability with the kernel PCA approach Recall the kernel PCA approach Project onto U, the top n left singular vectors Instead, project onto the top r left singular vectors Proceed as before Similar approach can be used for ITML The learned kernel is of the form n κA (xi , xj ) = κ(xi , xj ) + Sij κ(x, xi )κ(xj , y) i,j=1 Restrict S to be r × r instead of n × n, where r < n data points are chosen Rewrite optimization problem using this form of the kernel Constraints on learned distances are still linear, so method can be generalized Both approaches can be applied to very large data sets Example: ITML has been applied to data sets of nearly 1 million points (of dimensionality 24,000) Brian Kulis University of California at Berkeley Metric Learning
  93. 93. Nearest neighbors with Mahalanobis metrics Once metrics are learned, k-nn is typically used k-nn is expensive to compute Must compute distances to all n training points Recent methods attempt to speed up NN computation Locality-sensitive hashing Ball trees One challenge: can such methods be employed even when algorithms are used in kernel space? Recent work applied in computer vision community has addressed this problem for fast image search Brian Kulis University of California at Berkeley Metric Learning
  94. 94. Other non-linear methods Recall that kernelized Mahalanobis methods try to learn the distance function G ϕ(x) − G ϕ(y) 2 2 Chopra et al. learn the non-linear distance 2 GW (x) − GW (y) 2 GW is a non-linear function Application was face verification Algorithmic technique: convolutional networks[Chopra, Hadsell, and LeCun; CVPR 2005] Brian Kulis University of California at Berkeley Metric Learning
  95. 95. Other non-linear methods Setup uses relative distance constraints Denote Dij as the mapped distance between points i and j Let (xi , xj , xk ) be a tuple such that Dij < Dik desired The authors define a loss function for each triple of the form Loss = α1 Dij + α2 exp(−α3 Dik ) Minimize the sum of the losses over all triples Metric is trained using a convolutional network with a Siamese architecture from the pixel level Brian Kulis University of California at Berkeley Metric Learning
  96. 96. Other non-linear methods Brian Kulis University of California at Berkeley Metric Learning
  97. 97. Application: Learning Music SimilarityComparison of metric learning methods for learning music similarity MP3s downloaded from a set of music blogs After pruning: 319 blogs, 164 artists, 74 distinct albums Thousands of songs The Echo Nest used to extract features for each song Songs broken up into segments (80ms to a few seconds) Mean segment duration Track tempo estimate Regularity of the beat Estimation of the time signature Overall loudness estimate of the track Estimated overall tatum duration In total, 18 features extracted for each song Training done via labels based on blog, artist, and album (separately)[Slaney, Weinberger, and White; ISMIR 2008] Brian Kulis University of California at Berkeley Metric Learning
  98. 98. Application: Learning Music Similarity Brian Kulis University of California at Berkeley Metric Learning
  99. 99. Application: Object Recognition Several metric learning methods have been evaluated on the Caltech 101 dataset, a benchmark for object recognition set size Brian Kulis University of California at Berkeley Metric Learning
  100. 100. Application: Object Recognition Used the Caltech-101 data set Standard benchmark for object recognition Many many results for this data set 101 classes, approximately 4000 total images Learned metrics over 2 different image embeddings for ITML: pyramid match kernel (PMK) embedding and the embedding from Zhang et al, 2006 Also learned metrics via Frome et al’s local distance function approach Computed k-nearest neighbor accuracy over varying training set size and compared to existing results Brian Kulis University of California at Berkeley Metric Learning
  101. 101. Application: Object Recognition Brian Kulis University of California at Berkeley Metric Learning
  102. 102. Results: Clarify Representation: System collects program features during run-time Function counts Call-site counts Counts of program paths Program execution represented as a vector of counts Class labels: Program execution errors Nearest neighbor software support Match program executions Underlying distance measure should reflect this similarity Results LaTeX Benchmark: Error drops from 30% to 15% LogDet is the best performing algorithm across all benchmarks[Davis, Kulis, Jain, Sra, and Dhillon; ICML 2007] Brian Kulis University of California at Berkeley Metric Learning
  103. 103. Application: Human Body Pose Estimation Brian Kulis University of California at Berkeley Metric Learning
  104. 104. Pose Estimation 500,000 synthetically generated images Mean error is 34.5 cm per joint between two random images Brian Kulis University of California at Berkeley Metric Learning
  105. 105. Pose Estimation Results Method m k=1 L2 linear scan 24K 8.9 L2 hashing 24K 9.4 PSH, linear scan 1.5K 9.4 PCA, linear scan 60 13.5 PCA+LogDet, lin. scan 60 13.1 LogDet linear scan 24K 8.4 LogDet hashing 24K 8.8 Error above given is mean error in cm per joint Linear scan requires 433.25 seconds per query; hashing requires 1.39 seconds per query (hashing searches 0.5% of database)[Jain, Kulis, and Grauman; CVPR 2008] Brian Kulis University of California at Berkeley Metric Learning
  106. 106. Pose Estimation Results Brian Kulis University of California at Berkeley Metric Learning
  107. 107. Application: Text Retrieval[Davis and Dhillon; SIGKDD 2008] Brian Kulis University of California at Berkeley Metric Learning
  108. 108. Summary and Conclusions Metric learning is a mature technology Complaints about scalability in terms of dimensionality or number of data points no longer valid Many different formulations have been studied, especially for Mahalanobis metric learning Online vs offline settings possible Metric learning has been applied to many interesting problems Language problems Music similarity Pose estimation Image similarity and search Face verification Brian Kulis University of California at Berkeley Metric Learning
  109. 109. Summary and Conclusions Metric learning has interesting theoretical components Analysis of online settings Analysis of high-dimensional (kernelized) settings Metric learning is still an interesting area of study Learning multiple metrics over data sets New applications Formulations that integrate better with problems other than k-nn Improved algorithms for better scalability ... Brian Kulis University of California at Berkeley Metric Learning

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